This invention relates to an X-ray exposure apparatus used in the manufacture of various devices, namely semiconductor chips such as IC and LSI chips, display devices such as liquid crystal panels, detector elements such as magnetic heads and image sensing devices such as CCDs, to a reticle such as a mask and to a device manufacturing method using the reticle.
The recent increase in density and operating speed of semiconductor integrated circuits has been accompanied by a decrease in pattern-line width of integrated circuits. Semiconductor manufacturing methods also demand much higher performance. For this reason, steppers utilizing shorter and shorter exposure wavelengths, e.g., extreme ultraviolet rays such as KrF lasers (having a wavelength of 248 nm), ArF lasers (having a wavelength of 193 nm) and F2 lasers (having a wavelength of 157 nm) and X-rays (0.2xcx9c1.5 nm), have been developed for exposure apparatus used in the formation of a resist pattern in the lithography part of the semiconductor manufacturing process.
With exposure using X-rays, an X-ray mask serving as a reticle on which a desired pattern has been formed is brought into close proximity with a wafer serving as a resist-coated substrate, and the X-ray mask is irradiated from above with X-rays to transfer the mask pattern to the wafer.
A method of exposure using synchrotron light emission has been proposed for the purpose of obtaining high-intensity X-rays in such an X-ray exposure technique, and it has been shown that a pattern can be transferred with a wavelength of less than 100 nm. The synchrotron radiation light source, however, requires elaborate facilities. Though the source is effective in the production of semiconductor devices, it is not suitable for small devices used in prototypes, for example. Accordingly, there has also been proposed an exposure apparatus that employs an X-ray source which is small enough to be usable in prototypes and which generates X-rays of high intensity. One example is referred to as a xe2x80x9claser plasma ray sourcexe2x80x9d, as illustrated in the specification of U.S. Pat. No. 4,896,341. This apparatus irradiates a target with laser light from a laser to generate a plasma, and uses X-rays that are produced from the plasma. Another example generates a pinch plasma by electrical discharge in a gas, and produces X-rays from this plasma, as described in the Journal of Vacuum Science Technology 19(4), November/December 1981, p. 1190.
Though not of the proximity type, an X-ray source having a plurality of plasma X-ray emission points is proposed in the specification of Japanese Patent Application Laid-Open No. 9-115803.
Regardless of which light source is used, the resolution of the transferred pattern declines because diffraction is utilized in proximity X-ray exposure. The wavelength of X-rays is short and does not cause a decline in resolution. However, it has been found that the decline in resolution becomes a problem as the pattern to be transferred becomes extremely fine.
For example, X-ray intensity distribution on the surface of a wafer is as indicated by the solid line in FIG. 4. The curve is obtained as the result of calculation by Fresnel integration, in which the thickness of the absorbing body was 0.25 xcexcm, the spacing between the X-ray mask and wafer was 10 xcexcm and the mask was irradiated with perfectly collimated X-rays. The mask had a line-and-space pattern of transparent portion 90 nm/absorbing body 90 nm. A peak in X-ray intensity appears below the transparent portion and at other locations as well. When this pattern is transferred to a negative resist and then developed, the resist at locations where the X-ray intensity is greater than a fixed value remains after development and is resolved as a pattern. The fixed value is considered to be a slice level and is decided by the type of resist, development time, type of developing solution and temperature. In the case of a chemical amplification resist, the fixed value is decided also by the PEB (Post-Exposure Bake) conditions, namely temperature and time.
For example, FIG. 4 illustrates the result obtained by normalizing the X-ray intensity distribution on the wafer surface by the X-ray intensity below a sufficiently large transparent portion. It is believed that if the slice level is 1.0, the resist between X1 and X2 will remain after development. Though the width of the resist pattern is, accurately speaking, different from the size L12 (=X2xe2x88x92X1) of the optical image, it will be understood that they approximately coincide, with the value being 66 nm.
Next, the size of the transparent portion is gradually changed, the X-ray intensity distribution is calculated and the width of the resist pattern is found from the size of the optical image. This is indicated by the solid line in FIG. 6. Here the width of the transparent portion is plotted along the horizontal axis and the size of the resist pattern along the vertical axis. The slice level is changed to 0.8, 0.6 and 0.4, as indicated by the dotted line, broken line and dot-and-dash line, respectively.
However, it will be understood from FIG. 6 that there is a region of transparent portions in which the width of the resist pattern does not necessarily increase but decreases instead and a region in which there is no change in the width of the resist pattern as the mask pattern size, i.e., the size of the transparent portion, increases. This indicates that performing exposure using a mask consisting of a mixture of patterns having a plurality of sizes in these regions is difficult.
It will be appreciated from the foregoing that a region in which the resist pattern does increase in size exists, regardless of the fact that the mask pattern is enlarged in size and there is an increase in the amount of X-rays that arrive at the mask surface, thus making it difficult to transfer the mask pattern. The reason for this is as follows: In this region of mask pattern sizes, X-rays that have passed through the transparent portion collect in the diffraction peak and act in a direction that raises X-ray intensity and not in a direction that broadens the width of the X-ray intensity pattern. For example, FIG. 5 illustrates an X-ray intensity distribution in which the size of the transparent portion is 220 nm. However, when this is compared with an X-ray distribution (FIG. 4) in which the size of the transparent portion is 90 nm, it is seen that the peak intensity is 1.5 times higher in FIG. 5. Since the peak intensity increases more than the ratio of the transparent portion sizes, the width of the peak decreases rather than increases regardless of the fact that the transparent portion size increases.
If the X-ray dose for which the diffraction peak intensity has risen can be converted by some method to a direction that enlarges the width of the diffraction peak, i.e., if the X-ray intensity distribution can be defocused by a suitable amount, then it should be possible to enlarge the width of the resist pattern along with the size of the mast pattern.
As a means for achieving this, it is considered to change the shape of the mask absorbing body or the X-ray spectrum to thereby change the X-ray intensity distribution on the surface of the wafer. However, this method not only complicates the apparatus but also may make it impossible to obtain the desired pattern. A better method is desired.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Accordingly, an object of the present invention is to make it possible to enlarge the width of a resist pattern with an increase in the size of the mask pattern.
Another object of the present invention is to provide a proximity-type X-ray exposure apparatus and method in which controllable parameters can be increased and a more suitable resist pattern obtained.
According to the present invention, the foregoing objects are attained by providing an X-ray exposure apparatus which includes an X-ray source having a target and a laser light source for irradiating the target with laser light to produce X-rays by the generation of a plasma, wherein a reticle and a substrate disposed in close proximity to each other via a predetermined proximity gap are irradiated with the X-rays to transfer a pattern on the reticle to the substrate by exposure, characterized in that the X-ray source irradiates a plurality of positions on the target with the laser light during exposure.
The present invention further provides a device manufacturing method characterized in that when a target is irradiated with laser light to produce X-rays by the generation of a plasma, and a reticle and a substrate disposed in close proximity to each other via a predetermined proximity gap are irradiated with the X-rays to thereby transfer a pattern on the reticle to the substrate by exposure, a plurality of positions on the target are irradiated with the laser light during exposure. Means for irradiating the plurality of positions with laser light may comprise a mirror for changing the angle of the laser light by reflecting the laser light, which is for irradiating the target, or a plurality of laser light sources focused on different positions of the target.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.